The present invention is a process for preparing cyclic organohydrogensiloxanes.
Cyclic organohydrogensiloxanes such as cyclic methylhydrogensiloxane are useful as crosslinkers in silicone coatings and encapsulating materials used in the electronic industry and in release paper coatings and can be used as intermediates to form SiH functional siloxanes. In typical processes for preparing organohydrogensiloxanes a first step involves the hydrolysis of an organohydrogendichlorosilane to form an equilibrium mixture containing cyclic organohydrogensiloxanes and short-chain linear organohydrogensiloxanes. Generally, the weight percent of cyclic organohydrogensiloxanes in the equilibrium mixture is small in relation to the weight percent of linear organohydrogensiloxanes present. Consequently when demand for cyclic organohydrogensiloxanes is high, an excess of linear organohydrogensiloxanes may be produced.
U.S. Pat. No. 5,395,956 describes a process comprising contacting an organohydrogendichlorosilane with about a stoichiometric equivalent of water to form a hydrolyzate, diluting the hydrolyzate in an inert solvent and contacting it with an acidic rearrangement catalyst to effect formation of cyclic organohydrogensiloxanes. The catalyst is generally a heterogeneous catalyst used as a fixed-bed or stirred-bed. U.S. Pat. No. 5,395,956 says that the acidic rearrangement catalyst can be a homogeneous catalyst such as hydrogen chloride, sulphuric acid, or chlorosulfonic acid, but that such homogeneous acids are generally not preferred since they must subsequently be neutralized.
U.S. Pat. No. 5,247,116 describes a process for producing cyclosiloxanes by contacting siloxanes with a strong acid catalyst in the absence of an added solvent.